Body panels for automotive vehicles are currently being manufactured using a superplastic (high elongation) forming process applied to certain manganese-containing and magnesium-containing aluminum alloy sheet stock. At the present time, the sheet stock is a specially prepared, fine grain microstructure aluminum alloy 5083. AA5083 has a nominal composition, by weight, of about 4 to 5 percent magnesium, 0.4 to 1 percent manganese, a maximum of 0.25 percent chromium, up to about 0.1 percent copper, up to about 0.4 percent iron, up to about 0.4 percent silicon, and the balance substantially all aluminum. Copper, iron and silicon are present as difficult to eliminate impurities.
Generally, the alloy is chill cast into a large ingot about 700 millimeters in thickness and subjected to a long homogenizing heat treatment. The slab is then gradually reduced in thickness by a series of hot rolling operations to a strip in the range of four to eight millimeters, depending somewhat on the goal for the final thickness of the sheet, and coiled. The coiled strip is then heavily cold rolled, usually in stages with possible interposed anneals, to a final sheet thickness in the range of about one to three or four millimeters.
The result of the thermomechanical processing is a coil of smooth surface aluminum sheet stock, the microstructure of which has been severely strained. The sheet material is heated to recrystallize it to a strain relieved, fine grain microstructure (grains less than about ten micrometers) and to a suitable forming temperature, e.g., 450 C to 500 C. In this condition a sheet blank can be stretch formed into an article of complex shape with regions of high biaxial stretching.
While this specially processed AA5083 type material is very useful for making articles such as automobile body panels it is much more expensive than the heavier carbon steel sheet which has long been used in the same applications. The above identified United States patent application describes a more efficient casting and thermomechanical processing method of making sheet material of magnesium containing aluminum alloy for sheet metal forming. The disclosed method comprises continuously casting such aluminum alloy compositions to form a cast slab. The cast slab is hot rolled, coiled, annealed and cold rolled. After recrystallization the sheet material is readily formable by hot stretch forming processes. The entire disclosure of the above identified application is incorporated by reference into this application.
It is an object of this invention to modify the composition of the above described magnesium containing aluminum alloys with the addition of copper to provide a continuously cast alloy processed as described in the application to a sheet material with higher formability than the Mg containing alloys that have contained minimal amounts of copper as an impurity.